In order to increase multifunctionality of a programmable logic circuit and expand use thereof in electronic equipment or the like, downsizing of a switch interconnecting logic cells and also decrease of on-resistance of the switch are required. A variable resistance type switch using a variable-resistance element using precipitation of metal in an ion-conducting layer conducting a metal ion is known to have a smaller size and smaller on-resistance than a semiconductor switch. PTL 1 discloses a two-terminal variable resistance type switch, and PTL 2 discloses a three-terminal variable resistance type switch, respectively.
A two-terminal switch is configured to sandwich an ion-conducting layer between an active electrode supplying a metal ion to the ion-conducting layer and an inert electrode not supplying a metal ion. A connection between both electrodes is turned on/off by formation and disappearance of a metal bridge in the ion-conducting layer. The two-terminal switch has a simple configuration, is easy to manufacture, and allows downsizing on the order of nanometers. On the other hand, a three-terminal switch is configured to share active electrodes or inert electrodes of two two-terminal switches as a control terminal and has higher reliability in a switch operation compared with a two-terminal switch.
A porous polymer being disclosed in PTL 3 and having silicon, oxygen, and carbon as main components is preferable as an ion-conducting layer. An ion-conducting layer using a porous polymer can keep dielectric breakdown voltage high even when a metal bridge is formed, and therefore has excellent reliability in a switch operation.
A method of integrating a two-terminal variable resistance type switch into a semiconductor device and a method of integrating a three-terminal variable resistance type switch into a semiconductor device, in order to apply a variable-resistance element to a wiring selector switch in a programmable logic circuit, are disclosed in PTL 4 and PTL 5, respectively. According to PTLs 4 and 5, in a manufacturing process of a semiconductor device including a copper wiring, a variable-resistance element can be integrated with high density merely by adding two photomasks, with a configuration using the copper wiring also as an active electrode of the variable-resistance element.
At this time, there is a problem that, when an ion-conducting layer is directly formed on the copper wiring, a surface of the copper wiring is oxidized and leak current increases. NPL 1 resolves the problem by sandwiching a metal thin film between the copper wiring and the ion-conducting layer in such a way that the metal thin film is oxidized by oxygen in the ion-conducting layer and becomes a part of the ion-conducting layer. Further, NPL 1 discloses that, at this time, thermal stability of a metal bridge is enhanced and retentive strength of the metal bridge is improved by the metal being taken into the metal bridge, and current required for a transition from an on-state to an off-state does not increase.
Specific forms of a wiring selector switch in a programmable logic circuit include a crossbar switch in which switch elements are arranged at intersections of input lines and output lines. In a crossbar switch, at least one selection transistor needs to be provided for each switch element in order to suppress sneak current when signal transmission is performed and to select a switch element when a switch operation is performed. However, there is a problem that, due to a large area of the transistor, the switch element cannot be downsized even though a variable-resistance element itself is small.
PTL 6 discloses a three-terminal switch downsized by connecting a bipolar rectifier element in series to a control terminal of a variable-resistance element, in place of a transistor. A switch operation of the three-terminal switch is performed through the rectifier element, and switching current is limited by arriving current of the rectifier element. Further, the rectifier element suppresses sneak current into an adjacent switch through the control terminal, and false switching of the adjacent switch can be prevented.
However, the method in PTL 6 has a problem that a plurality of switch elements existing on an input line or an output line in the crossbar switch cannot be turned on for supporting multi-fan-out. The reason is that current flows into an input line or an output line through a switch element being previously turned on, and false switching occurs at a switch element separate from a switch element intended to be turned on.
Then, PTL 7 discloses a four-terminal switch allowing support of multi-fan-out by connecting one rectifier element in series to each of two two-terminal switches constituting a three-terminal switch. The four-terminal switch suppresses sneak current and improves selectivity of a switch element when a switch operation is performed. PTL 7 also discloses a rectifier element improving a rectifying characteristic by providing a buffer layer specified with a work function and a relative dielectric constant on a rectifying layer of the rectifier element.